Regulation by external pH and stationary growth phase of the acetolactate synthase from Synechocystis PCC6803

Several characteristics identify the protein encoded by the alsS gene [sll1 981 in Cyanobase (http://www.kazusa.or.jp/cyano/cyano.html)] of Synechocyst is PCC6803 as an acetolactate synthase. The AlsS protein is about 60% homol ogous to the AlsS from Bacillus subtilis or other bacteria. These enzymes c ondense two pyruvates to form acetolactate, implicated in pH homeostasis vi a the acetoin-2,3-butanediol pathway or in valine biosynthesis. Transcripti onal fusions revealed that alsS was induced at the onset of stationary phas e, as in B. subtilis, a situation leading to an increase in the pH(out) to above 11 in Synechocystis. This is the first cyanobacterial gene showing a dependence on pH for its expression. Induction was also obtained by the pre sence of > 100 mM Na+, the effect being prevented by amiloride, in agreemen t with Na+/H+ exchange in the pH homeostasis process. Homology of the Synec hocystis AlsS protein to the close family of acetohydroxy acid synthases (i ncluding one in Synechocystis) is around 30%. These enzymes are involved in the parallel routes for valine/leucine and isoleucine biosynthesis. No phe notype of auxotrophy for any of these amino acids was associated with a nul l mutation in the Synechocystis alsS gene. The AlsS enzyme did not compleme nt the isoleucine deficiency of an acetohydroxy acid synthase-deficient Esc herichia coli mutant.